Conventionally, various types of drive devices are known such as those disclosed in Patent Documents 1 and 2. These drive devices are provided with an electromagnetic clutch mechanism accommodated in a housing. The electromagnetic clutch mechanism is provided with an armature, a rotor, and a coil. A worm wheel is arranged around a rotary shaft rotatably supported by the housing so as to rotate relative to the rotary shaft, and the armature is arranged on one side of the worm wheel in the axial direction and coupled to the worm wheel so as to integrally rotate with the worm wheel. The rotor is arranged to face the armature, supported on the rotary shaft so as to integrally rotate with the rotary shaft, and capable of making frictional engagement with the armature when electricity is supplied to the coil.
Furthermore, an annular magnet is fitted into the outer circumference surface of the rotor. A sensor for detecting a polarity change of the magnet in association with the rotation of the rotor is arranged to face the magnet. The rotor axially coincides with the magnet in the axial direction, by which the electromagnetic clutch mechanism is made small in the axial direction.
When abrasion powder produced on actuation of the electromagnetic clutch mechanism is scattered toward a circuit substrate including the sensor, the abrasion powder may result in a short-circuit breakdown of the sensor. Furthermore, for example, an electronic control unit which is integrally accommodated into a housing is employed, this constitution may result in a short-circuit breakdown of the electronic control unit on which important and delicate components such as a microcomputer are mounted.    Patent Document 1: Japanese Published Laid-Open Patent Publication No. 2000-179233    Patent Document 2: Japanese Published Laid-Open Patent Publication No. 2003-74255